Development Of Macromolecular Prodrugs Of Bet-Bromodomain Inhibitors With Superior Anti-Tumor Efficacy That Are T-Cell Sparing And Devoid Of Systemic Toxicity

Small molecule BET inhibitors are promising anti-cancer agents, but their clinical development has been limited by hematological and gastrointestinal (GI) toxicity. For the prototype benzodiazepine-derived inhibitor (OTX-015), the dose-limiting toxicities (DLTs) are thrombocytopenia (96%), anemia (91%), and neutropenia (51%) with additional GI events (diarrhea, vomiting and mucositis) reported to limit patient compliance despite evidence of durable/objective tumor responses. Accordingly, based on a review of entries in www.clinicaltrials.gov from January 2017 to January 2018, 11 out of 12 programs are reporting protracted phase I/II development times and reduced patient enrollment targets. To improve the narrow therapeutic index of current BET inhibitors, we here report the development of macromolecular BET inhibitor prodrugs with a favorable bio-distribution and release of active drug in tumor compared to other tissues, including gut and bone marrow. We successfully conjugated two structurally distinct BET inhibitors, including the OTX-015, to Brush polymers using an array of different linker chemistries and evaluated them for in vivo efficacy and toxicity using immunocompetent, orthotopically implanted mouse tumor models. Through rational design of drug conjugation and linker chemistry, we optimized the PK properties and drug release rates offering a ‘depot-like9 release of drug in tumor tissue resulting in both improved efficacy and reduction of systemic dose-limiting toxicity. Specifically, these novel formulations were evaluated for myelosuppression and GI toxicity using an array of in vitro, clinical pathology and immunohistopathology techniques. Compared to non-conjugated BET inhibitors, which showed dose-dependent body weight loss, diarrhea, and suppression of white blood cells, the macromolecular BET-BRUSH prodrugs spared the lymphocytes, platelets and neutrophils and showed minimal suppression of the reservoir of myeloid cells in the bone marrow. The improved therapeutic index of the BET-Brush compounds was confirmed through detailed PK/PD/Efficacy studies correlating the concentration of both released and polymer-bound BET inhibitor in tumor and plasma with quantitative tissue biomarker modulation (c-MYC, HEXIM-1 and CD180). Whole organ bio-distribution studies using fluorophore-conjugated BET-Brush confirmed the favorable distribution into tumor over the gut and bone marrow, with BET-Brush showing profound modulation of biomarkers in tumor tissue, but not gut. Notably, the BET-Brush compounds showed suppression of PD-L1 expression in tumors, which in context of preserved T-cells, can make BET-Brush a promising combination with immuno-oncology therapy. Paired with an excellent safety profile of the polymer backbone in rat and non-human primates, these data support the further development of BET-Brush prodrugs as an infrequently dosed treatment for human cancers. Citation Format: Farrukh Vohidov, Jannik N. Andersen, Kyriakos D. Economides, Michail V. Shipitsin, Olga Burenkova, Nolan M. Gallagher, Peyton Sheih, Matthew Golder, Jenny Liu, William K. Dahlberg, Hung V. Nguyen, Deborah J. Ehrlich, Julie Kim, Sung Jin Huh, Bhavatarini Vangamudi, Allison M. Neenan, James C. Ackley, Joelle Baddour, Sattanathan Paramasivan, Gaurab KC, David J. Turnquist, Jenny K. Saucier-Sawyer, Paul W. Kopesky, Samantha W. Brady, Michael J. Jessel, Lawrence A. Reiter, Donald E. Chickering, Jeremiah A. Johnson, Peter Blume-Jensen. Development of macromolecular prodrugs of BET-bromodomain inhibitors with superior anti-tumor efficacy that are T-cell sparing and devoid of systemic toxicity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-062.